1. Field of the Invention
The present invention relates to an antenna diversity communications device for communicating using frequency hopping and antenna diversity techniques with a plurality of antennas.
2. Description of the Related Art
In the field of digital wireless communications, spread-spectrum mode has come to be used widely in recent years. One of the techniques for spread-spectrum communications uses frequency hopping technique. Frequency hopping is a communications mode in which the transmitting frequency is not fixed at a specific frequency but switched from one frequency to another in coded sequence.
It is a common phenomenon digital wireless communications creates a multiple-wave transmission path where received reflected waves and diffracted waves overlap direct waves to produce fading due to the synthesis of multiple waves with different arrival times, thus creating transmission errors.
The spread-spectrum communications mode is a countermeasure against fading due to such a multiple-wave transmission path, in which the Direct Sequence (DS) mode and the Frequency Hopping (FH) mode are well-known.
The frequency hopping mode switches (hops) the center frequency of the transmitted signal in a prescribed sequence to spread the spectrum of the transmitted signal over a substantial bandwidth.
If the hopping speed is higher than the information speed, the technique is called a fast frequency hopping mode. If the hopping speed is lower than the information speed, the technique is called a low speed frequency hopping mode.
In fast frequency hopping, a single information symbol is transmitted over a plurality of frequencies. In low speed frequency hopping, a plurality of information symbols are transmitted while the transmitter remains at a single frequency.
The low speed frequency hopping mode can be constituted with simpler devices compared to the fast frequency hopping mode. Thus, the low speed frequency hopping mode is more often employed in systems such as a wireless LAN since a fading countermeasure effect can be obtained using such techniques because resending of missed items and error correction codes are sufficient, although transmission efficiency is sacrificed.
Antenna diversity, on the other hand, is a mode in which signal fading at an antenna is reduced by using a plurality of antennas with low fading correlations. The signals from the antennas are switched to the receiver depending on the levels of signals at the antennas. This method synthesizes a single received signal from the plurality of antennas.
Antenna diversity can be divided into several modes:                a) space diversity using a plurality of antennas spatially separated from each other;        b) polarization diversity using a plurality of antennas of different polarization; and        c) radiation pattern diversity using a plurality of antennas of different directionality.        
The present invention relates to antenna diversity.
The combining method in an antenna diversity system can be divided into several methods:                a) the maximum ratio combining method which equalizes each received signal phase and synthesizes received signals by weighting with SN ratios of each received signal;        b) the equal gain combining method which equalizes each received signal phase and synthesizes them as they are; and        c) the selective combining method which selects a signal having the maximum reception level of all received signals.        
The combining method can also be divided into a method of combining before detection and a method of combining after detection according to the manner in which the phase of receiver combining is conducted. However, both of these combining methods require such a plurality of receivers that the hardware size is impractically large.
The antenna switching method is simpler. The antenna switching method receives on a single receiver while switching the input between a plurality of antennas. Switching from one antenna to another is performed when the signal level at the receiver drops below a prescribed switching level.
There are two operational algorithms for antenna switching when the signal level from antenna drops below the prescribed signal level:                a) the SE (Switch-and-Examine) mode continues to search outputs of antennas looking for the one providing the highest signal level;        b) the SS (Switch-and-Stay) mode that switches from antenna to antenna until it finds one providing a signal above the switching level. It then remains with that one antenna until the received signal level falls below the switching level. At that time, the next switching operation begins to find another antenna providing a signal exceeding the switching level.        
A special problem arises with the antenna switching method used in a mobile station of a digital cellular telephone system using the TDMA (Time Division Multiple Access) mode which is in use in Japan. This problem requires detecting and comparing the reception levels of two antennas within a time slot signal interval immediately before reception of a signal of the local time slot, and then switching to the antenna providing a higher level.
Referring to FIG. 12, a conventional antenna diversity communications device includes two antennas 101 and 102 and the switching unit 103 that selects one or the other of the antennas 101 and 102.
An LNA (Low Noise Amplifier) 104 amplifies the signal from the antenna selected by the switching unit 103 and applies the amplified signal to a mixer 105. A local oscillator signal generated by a local oscillator 106 is applied to the mixer 105. The mixer 105 mixes, or heterodynes, these signals to produce an IF (Intermediate Frequency) signal. The IF signal is applied to an AGC (Automatic Gain Control) 107. The AGC 107 amplifies the inputted signals with a variable gain to maintain a substantially constant amplitude output, and outputs the result to a detector.
A RSSI (Received Signal Strength Indicator) 108 monitors the gain-control signal from the AGC to measure the signal intensities of the signals from the antennas 101 and 102. The output from the RSSI 108 is applied a comparing unit 109, which compares the intensities of the signals from the antennas 101 and 102.
Referring momentarily to FIG. 13, the intensity comparison is conducted during a time slot immediately prior to a local slot. The switching unit 103 switches to the one of antennas 101 and 102 providing a higher intensity signal. This switching takes place in the time slot immediately prior to the local time slot. By constantly choosing the output of the antenna producing the higher output, the effects of fading are minimized, thereby taking advantage of the antenna diversity effect. By switching the antennas before the local time slot, switching noise remains isolated from the output signal.
The above prior-art device is difficult to use while simultaneously using the frequency hopping mode. In the frequency hopping mode, the signal is a narrow band signal when viewed within a single hopping frequency. Although the signal is affected by fading from the viewpoint of each hopping frequency, the frequency diversity effect can be obtained in the high-speed frequency hopping mode by transmitting a single information symbol on a plurality of frequencies.
On the contrary, the frequency diversity effect is difficult to achieve in the low speed frequency hopping mode because a plurality of information symbols are transmitted on a single frequency. Although the transmission quality can be improved using such techniques as resending missed portions and error correction codes, transmission efficiency drops.
To cope with these problems, the antenna diversity mode is used with the frequency hopping mode at the same time, in particular, the low speed frequency hopping mode, has been conceived. This has proven effective for mobile stations, etc., for using the antenna switching mode that permits the use of simple hardware and processing.
However, the following problems exist in using the antenna diversity mode by the antenna switching method with the frequency hopping mode at the same time.
In the following description, Bluetooth is used as an example of a digital wireless communications system using low speed frequency hopping mode. The present invention can be applied as long as it does not deviate from its purpose using a system other than Bluetooth.
Bluetooth is a wireless communications standard for short distance prepared by its standardization organization, Bluetooth SIG (http://www.bluetooth.com). An outline of the system is reported by Jaap C. Haartsen in the publication, “Bluetooth™: A new radio interface providing ubiquitous connectivity,” IEEE Vehicular Technology Conference, Vol., 1, pp 107–111, May 2000,” etc.
In Bluetooth, one time slot is 625 μsec. Frequency hopping basically occurs once per time slot. The hopping speed is 1600 hop/sec.
One packet is transmitted during each hop. There is thus no possibility that frequency hopping can occur during transmission of a packet.
Variations in reception level during reception of a packet are negligibly small under busy conditions such as being stationary or walking. Therefore, it is sufficient to switch the antenna once per hop (switching at hopping frequency) when the antenna switching mode is used as an additional mode.
However, since the immediately preceding time slot has a different hopping frequency, the antenna diversity effect cannot be obtained by simply applying the antenna switching mode consisting of detection, comparison and switching of each antenna's reception level to the aforementioned SE and SS modes or in the immediately preceding time slot.
Further, if an antenna is switched by detecting and comparing the reception level of the antenna within the reception time slot, the signal will be affected by switching noise.